The mechanism of the phosphorylation-dependent myosin-linked regulation of smooth muscle and nonmuscle myosins is being investigated. This involves several approaches: (1) measurement of rate constants for various steps in the kinetic cycle for the hydrolysis of MgATP by heavy meromyosin (HMM), the proteolytic subfragment of myosin, in the presence and absence of actin; (2) preparation of HMM from cytoplasmic myosins and characterization of their kinetic properties, and (3) use of an in vitro motility system to quantitate how the velocity of movement of myosin-coated beads is affected by various factors. We are also studying the mechanism of action of caldesmon, a possible regulatory protein associated with actin filaments.